This invention relates to a polymerization process. In a more specific aspect, the invention relates to a method of recovering the diluent employed in the polymerization process.
In many polymerization processes for the production of normally solid polymer, a stream is formed which is a slurry of particulate polymers suspended in a liquid medium, ordinarily the reaction diluent. Typical examples of such processes are disclosed in Hogan and Banks' U.S. Pat. No. 2,285,721 and in G. T. Leatherman and C. V. Detter's U.S. patent application Ser. No. 590,567, filed June 11, 1956, and now abandoned. The disclosures of that patent and that application are incorporated herein by reference. While the polymerization processes described in those documents employ a catalyst comprising chromium oxide and a support, the present invention is applicable to any process producing an effluent comprising a slurry of particulate polymer solids suspended in liquid diluent. Such reaction processes include those which have come to be known in the art as particle form polymerizations.
In order to minimize costs in most commercial processes, it is desired to separate the polymer and the diluent in such a manner that the diluent is not exposed to contamination so that the diluent can be recycled to the polymerization zone with minimal, if any, purification. Some typical diluent recovery processes are disclosed in the Scoggin et al U.S. Pat. No. 3,152,872, the disclosure of which is incorporated herein by reference. In those processes, a slurry of solid polymer and diluent taken from the polymerization zone is passed into a flash zone wherein a substantial portion of the diluent is separated from the polymer. The thus separated diluent is condensed and purified for reuse if desired. The polymer solids having residual amounts of diluent contained thereon are passed from the flash zone to a conveyor dryer zone wherein a portion of the diluent is vaporized and passed back to the flash zone. The polymer solids still carrying residual amounts of diluent are then passed to a purge zone wherein a heated non-combustible gas is used to remove the diluent. In order to minimize loss of diluent, there has generally been an inert gas seal provided between the dryer and the purge zone. In the past, the vaporized diluent from the purge zone has been vented or flared. To minimize operating costs and hydrocarbon emissions, it is increasing important to recover even the diluent from the purge step. It has been estimated that the amount of diluent lost through flaring of the purge zone effluent could be as high as 0.03 pounds per pound of polymer. An object of the present invention is to provide a particularly useful process for recovering the diluent from such purge zones.